Liquid chromatography (“LC”) is a method by which various species from a complex mixture can be separated out into their individual components. The individual species or components will elute from the liquid chromatography system at substantially different times.
A liquid chromatography system used in conjunction with a mass spectrometer (“MS”) or a tandem mass spectrometer (“MS/MS”) represents a powerful analytical instrument. However, the relatively short time that any particular analyte species is present in the ion source of the mass spectrometer limits the quality of data that can be achieved. Similarly, the length of time that any particular species of parent ion is present in the ion source limits the number of different MS/MS product ion mass spectra that can be measured. This length of time is determined by the peak elution profile for the particular liquid chromatography system being used.
Advances in the speed of mass spectrometry analysis have enabled a greater number of quality MS and MS/MS experiments to be performed during the time scale of a liquid chromatography peak. However, limitations still exist as the peak elution time can be faster than the time required to perform all of the desired MS and MS/MS experiments. These limitations often result in compromises in the quality and/or quantity of experimental data produced. Furthermore, the on-going development of faster liquid chromatography systems for increased chromatographic resolution and faster analysis has compounded these problems.
U.S. Pat. No. 7,797,988 (Schultz) discloses a method wherein the eluent eluting from an LC column is split. One portion of the eluent is sent to a mass spectrometer for analysis, while at the same time the other portion is stored in a storage tube. Once the analysis of the first portion is complete, the stored portion of eluent is sent to the mass spectrometer for analysis. In this method, each chromatographic peak is introduced into the mass spectrometer twice, thereby theoretically doubling the time available for mass spectrometry experiments.
However, using this method it can be difficult to precisely predict when a delayed chromatographic peak corresponding to a particular chromatographic peak of interest will be introduced to the mass spectrometer and it can therefore be difficult to exploit the delayed peak fully (i.e. by performing the desired additional experiments at the appropriate time). This uncertainty is mainly due to the relatively long delay times inherent in the method (which are typically very long when compared to a single chromatographic run), but additional uncertainty is also introduced by unpredictable diffusion/mixing effects that arise when the flow of the eluent is stopped.
It is also known to attempt to effectively extend the time that a peak elutes by reducing the flow rate through a liquid chromatography system when species of interest are identified by a mass spectrometer. This technique is known as peak parking or variable flow chromatography. For example, US 2006/0186028 (Micromass) discloses an arrangement wherein separate pumps are used to effect variable flow chromatography. These arrangements do not suffer from uncertainty in the delay time because a peak of interest is simply extended. However, these arrangements require accurate control of the liquid chromatography flow rate and can therefore be relatively complex. Varying the liquid chromatography flow rate can also affect the chromatographic performance and chromatographic resolution.
It is therefore desired to provide an improved analytical instrument and an improved method of analysing a sample.